The present invention generally relates to power system protection, and more specifically, to an apparatus and method for providing differential protection for a phase angle regulating transformer in a power system.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include protective devices and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
Protective devices and procedures act to isolate some power system element(s) from the remainder of the power system upon detection of the abnormal condition or a fault in, or related to, the power system element(s). Logically grouped zones of protection, or protection zones utilizing the protective devices and procedures, are established to efficiently manage faults or other abnormal conditions occurring in the power system elements.
In general, protection zones may be classified into six types including: (1) generators and generator-transformer elements (2) transformers, (3) buses, (4) lines (transmission, sub-transmission and distribution), (5) utilization equipment (motors, static loads), and (6) capacitor or reactor banks. Portions of the protection zones typically overlap each other to provide redundancy and to ensure that faults and their locations are properly identified. Thus, each protection zone normally includes protective relays that provide backup for the relays protecting the power system elements of adjacent protection zones. As a result, a variety of protective devices are required. Such protective devices may include different types of protective relays, surge protectors, arc gaps and associated circuit breakers and reclosers.
Although the fundaments of power system protection are similar, each of the six types of protection zones use protective devices that are based on the characteristics of the power system elements in that category. More specifically, different protective relays utilizing a variety of protective schemes (e.g., differential current comparisons, magnitude comparisons, frequency sensing), are required to protect the various power system elements. For example, a differential relay, having nn electrical connections (via current transformers), is designed to monitor current flowing into, for example, a power bus (i.e., a protection zone) by measuring the current flowing into the bus from each terminal to the bus and calculating inter alia, the sum of all measured current, or the operate current. As is known, when the bus is operating under normal conditions, the sum of all of the (primary) currents entering the bus is about zero (Kirchhoff's current law). If the bus has a short circuit, or is faulted, its operate current will be substantially different from zero, indicating that there is some impermissible path through which a current is flowing. If the operate current exceeds some threshold, or pickup current, the differential relay issues a trip signal to an associated power circuit breaker(s) causing it to open and isolate the faulted bus from the remainder of the power system.
Because of potential relay mis-operation, one type of differential relay is designed with a restraint mechanism intended to restrain the differential relay (e.g., prevent it from issuing a trip signal) under certain circumstances. One restraint mechanism includes increasing the pickup current of the current differential relay as the currents entering the protected element increase; in other words, as the restraint current increases, the operate current required to cause a trip increases. Such differential relays are often referred to as percentage-restrained differential relays. For example, Equation (1) illustrates one example of calculating the operate current for a current differential relay that utilizes a restraint mechanism. Alternate schemes may also be used.Ioperate>k·Irestraint^Ioperate>min pu   (1)where Ioperate=|Ī1+Ī2+Ī3+ . . . Īn| or the phasor sum of currents flowing in the protection zone, and Irestraint=(|Ī1|+|Ī2|+|Ī3|+ . . . |Īn|)/p or some measure of the current flowing through the protection zone, and k=constant, min pu=constant, p=constant
Thus, the percentage-restraint principle provides security by requiring that the operate current exceeds some percentage of a measure of the current flowing through the protection zone before the differential relay may operate. This requirement that the operate current exceeds a percentage of the restraint current allows the differential relay to tolerate low levels of mismatch in the current measurement at each boundary of the protection zone. The same characteristic also allows the differential relay to tolerate false differential current caused by CT saturation (i.e., where the core becomes saturated, the magnetic flux ceases to change with the primary current and the secondary current is no longer a multiple of the primary current). In addition, because the differential relay operates on the difference current rather than only on the through current in the system, it is highly sensitive.
Differential relays are designed to measure currents no greater than 100 amps via nn electrical connections, and as a result, the differential relays are coupled to the protection zone via a number of current transformers. The current transformers operate to proportionally step-down the primary power system current flowing into the protection zone (while retaining the same phase relation), to a magnitude that can be readily monitored and measured by the differential relay. The resulting lower secondary currents can be filtered, sampled, etc., by the differential relay to determine corresponding phasors representative of the primary current flowing into the protection zone. The phasors are then used in the differential logic scheme (as well as other logic schemes such as an instantaneous overcurrent scheme) of the differential relay. Accordingly, the protection zone is determined by the location of the current transformers that define the differential zone.
Because of their ability to tolerate mismatch in the current measurements, the percentage-retrained differential relays are often used for protection zones that include a power transformer(s). Not all power transformers however operate to step-up or step-down voltage. One type of transformer, a phase angle regulating (PAR) transformer, which is also known as a phase shifting transformer (PST) or “power flow controller”, is typically included in the power system to introduce a phase shift between three-phase voltages at two (parallel) buses. These busses are connected by a transmission line to affect the flow of active power by inserting an out-of-phase voltage in series with the voltage of the bus(es), under load. On-load tap changers (LTC) are used to introduce such a phase shift, or out-of-phase voltage. As a result, the relative loading in the line can be changed. A tap changer control mechanism of the PAR transformer monitors the power flow and adjusts the position of the LTC to add or subtract degrees of phase shift to maintain a predetermined set point power flow (e.g., 102 MW).
The PAR transformer may be used to provide the variable (almost linear) phase shift as a function of the LTC position. For example, a delta-hexagonal phase angle regulating (PAR) transformer includes 3 pairs of parallel windings arranged in a hexagonal shape, where each pair corresponds to a phase, and where a 1:1 voltage is maintained while the phase shift is varied via the LTCs. Each pair includes one tapped winding with two load tap changers, and one untapped winding. The two LTCs vary phase shift by moving transformer input (Source) terminals (S1, S2, S3) and output (Load) terminals (L1, L2, L3) symmetrically with respect to the middle tap to cause the output voltages VL1, VL2, VL3 to lead or lag the input voltages VS1, VS2, VS3 by up to 32.9 degrees. Other types of PAR transformers include delta secondary winding/grounded wye exciting windings connections, wye secondary series winding/delta primary exciting winding connections, and tapped series winding design, to name a few.
As noted above, differential elements are often used to provide power transformer protection. Unfortunately, unlike application to typical power transformers, application of differential elements to PAR transformers is often difficult due to the continuously variable phase angles of the measured currents between the source- and load-side of the PAR transformer. In fact, a small difference in phase angle may translate into a large operate current thereby rendering the differential relay unsuitable for its intended application.
Traditional methods of addressing the fixed phase shift across a transformer have included combining currents in the differential circuit to mimic combinations of currents in the transformer. With the PAR transformer however, the amount of current contributed from the (two) other phases and combined into each phase varies with its associated tap position. Thus, under external (through-fault) conditions, the currents entering and exiting the transformer cannot be easily balanced thereby causing the differential elements to incorrectly trip for an external fault. Thus, traditional transformer differential protection cannot be effectively applied to a PAR transformer.